Generally, cellulose triacetate film is employed for a film such as a protective film for a polarizing plate or an optical by functional film, and a solvent casting process produces the cellulose triacetate film. In the solvent casting process, a dope being a polymer solution consisting of a cellulose acetate with high concentration prepared by mixing and dispersing a cellulose acetate and a solvent, is continuously cast from a casting die onto a casting support, stripped off after being dehumidified to some extent on the casting support, dried and wound to obtain a roll of the film.
By the way, at the timing of casting the dope from the casting die onto the casting support, an uneven surface drying defect by a sojourn of the dope may be formed on the die lip end. The uneven surface drying defect tends to form particularly at both lateral edge portions of the cast dope (the uneven surface drying defects at both side edge portions are referred to as slugs at lateral edge portions below). In the case where the uneven surface drying defect forms on the die lip end, streaks would appear beginning at the uneven surface drying defects thereby degrading the surface quality of the film.
Further, in the case where the slugs are formed at both lateral edge portions of the cast dope, they contact with a web of dope after being cast from the casting die and before it impinges to the casting support (referred to as “bead” below), thereby making problems of causing thickness unevenness at the lateral edge portions of the film, or causing remained dope after the stripping off of the film from the casting support, and causing the generation of a tearing point at the lateral edge portions of the film. There is another problem that the slugs may leave off from the dope and hung on a labyrinth part along the way before the stripping off until contacting with the film thereby causing a breaking down of the film.
Accordingly, various countermeasures for preventing the slugs at lateral edge portions are conventionally proposed particularly with regards to both side edge portions. For example, U.S. Pat. No, 3,112,528, JP 2-208650A, JP 5-86212A, etc. disclose a method of dripping a solvent to both lateral edge portions of a discharge lip of a casting die, or a method of depositing a mixture of vapor of the solvent and air to both lateral edge portions of a discharge lip of a casting die.
However, these conventionally proposed methods for preventing the slugs at lateral edge portions were not sufficiently effective depending on the characteristics of the dope because they are based on the principle of inhibiting the bad effects of the slugs at lateral edge portions by dissolving them again.
For example, JP 2-208650A and JP 5-86212A proposed as shown in FIG. 3 providing a feed pipe 32 for feeding a solvent at both lateral edges of a casting die 31, and by feeding the solvent 33 from the feed pipe 32 at both lateral edges of a bead 34, aiming to prevent the slugs at lateral edge portions. However, these conventional methods were unable to completely prevent the slugs at lateral edge portions, and the slug 35 may often formed on both lateral edge portions of the bead. By the way, numerical symbol 36 shows an impinging point of the bead with the casting support.